Polymerization reactions usually lead to products of varying polydispersity, which means that such polymeric products contain a range of components, from low to high molecular weight. The quality of a final polymeric product (polymer for short) to a large extent depends on how broad its molecular weight distribution is. Usually, the broader the distribution, the lower the value. Hence, the common challenge in polymer manufacturing is to make the molecular weight distribution as narrow as possible. Since controlling the polymerization reaction conditions has only a limited effect on the molecular weight distribution, a separation process called fractionation is needed to narrow down the polymer weight distribution by separating either the light and/or heavy fractions from the bulk product, downstream of the polymerization reactor.
The fractionation approaches disclosed in the prior art (see U.S. Pat. No. 3,969,196; U.S. Pat. No. 3,294,772; U.S. Pat. No. 2,457,238; Krukonis, V. POLYMER NEWS, 11, 7-16, 1985; McHugh, M.A. and Krukonis, V.J. SUPERCRITICAL FLUID EXTRACTION: Principles and Practice, Butterworths, 1986, pages 143-180; Kumar, S.K. et al. FLUID PHASE EQUILIBRIA, 29, 373-382, 1986; Kumar, S.K. et al. MACROMOLECULES, 20, 2550-2557, 1987; McHugh, M.A. and Krukonis, V.J. in Encycl. Polym. Sci. Eng. 16, 368-399, 1989) are based on a general concept of using supercritical fluids as solvents, but none specifically discloses the use of mixed solvents for polymer fractionation. In the present invention, the solvent is a supercritical fluid containing at least two components, antisolvent and cosolvent. The key advantage of this invention is a higher degree of flexibility in selecting separation conditions and, hence, a better extraction efficiency.